1. Field of the Invention
This invention relates to a hemming apparatus, and more particularly to the drive mechanism for causing a hemming tool to form a hem between a pair of overlapped metal pieces at the peripheral part of a workpiece.
2. Description of the Prior Art
Hemming is a technique that is widely used in the automotive industry for joining a sheet of metal that serves as an external body component to a formed piece of metal that serves as a reinforcing element for the external body component. It is also used in the white goods industries for seaming two pieces of sheet metal together. As an example, a door of most automotive vehicles is of a two-piece construction in which the outer edge of the outer element of the door is folded over against the outer edge of an inner reinforcing element by a hemming process. Devices for performing hemming operations of the type described are shown in U.S. Pat. No. 4,706,489 to Dacey, Jr., and U.S. Pat. No. 4,901,555 to Shimoichi.
Hemming processes as heretofore described generally utilize an outer element with the outer edge prefolded in the form of a flange to lie approximately perpendicularly to the main portion of the outer element, such prefolding being done most conveniently in the stamping operation that is customarily utilized in the forming of such outer element. The hemming of the flange requires that it be folded over from the prefolded condition approximately 90 degrees, to be against the outer edge of the inner element, after the inner element, whose main portion extends generally parallel to the main portion of the outer element, has been placed inside the flange of the outer element. The folding over or hemming of the flange of the outer element in many hemming processes of the prior art is done in multiple stages, usually in two stages. In a first stage, force is applied generally perpendicularly to the original orientation of the flange to cause it to bend approximately 35.degree. to 55.degree. from its original orientation. In the second stage, force is applied generally parallel to the original orientation of the flange to cause the partially bent flange to bend an additional approximately 55.degree. to 35.degree. to complete the approximately 90.degree. of folding of the flange from its prefolded condition to securely engage the outer edge of the inner element of the two-piece structure that is being hemmed.
An illustration of the sequence of a two-stage hemming operation is shown in FIG. 10. It can be seen that a workpiece comprises an inner element overlying an outer element adjacent an upstanding flange of the outer element. The workpiece rests on a bed of the apparatus, typically against a brace. A hemming tool is moved horizontally to a first position over the bed, and then drawn towards the bed in the first stage to an operative position where the tool engages the upstanding flange causing it to bend preliminarily from its original orientation. The tool is then withdrawn, and moved horizontally to a second-stage position, whereupon it is again urged vertically toward the bed to the operative position where the second-stage force is applied to the flange to complete the hemming operation.
It has been recognized that there are advantages with performing an entire hemming operation in a single set of tooling, as disclosed in U.S. Pat. No. 3,191,414 to Kollar, et al., and in U.S. Pat. No. 3,276,409 to St. Denis. The Kollar et al. patent describes a hemming tool that is actuated sequentially in horizontal and vertical directions by separate hydraulic cylinders acting through a linkage system. Such hydraulic structures are typically massive, costly, and complex. It has been found advantageous to use pneumatic systems for driving hemming tools. E. R. St. Denis & Sons Limited manufactures typical air-operated hemming machines.
Existing pneumatic hemming machines utilize an upper steel which supports the hemming tool, and which is urged toward the bed by means of a draw rod. The draw rod is connected, usually by a lever, to the shaft of a pneumatic cylinder, the lever serving to amplify the force of the pneumatic cylinder. One of the disadvantages of existing pneumatic hemming machines is a limitation on the amount maximum force which can be delivered to the hemming tool for application to the workpiece. However, since a typical hemming station requires the use of several hemming tools arranged end to end around the perimeter of the parts that are joined to one another, there is a limit to the size and weight of the driving mechanism that can be accommodated for each tool.